This invention relates to contact lenses and, more particularly, to contact lenses having bifocal characteristics.
Bifocal contact lenses have previously been produced. Such lenses generally follow the design of bifocal lenses employed in conventional eye glasses in that the lenses usually restrict the two focal powers to separate localized areas within the lens. The major problem inherent in these designs, is the difficulty encountered when the wearer tries to shift his eye position between the distance viewing portion of the lens and the near viewing portion of the lens. In order for this to be accomplished, the lens must be stabilized in such a manner as to allow the eye to move relative to the contact lens in a reasonably precise fashion. This has proved to be very difficult.
The present invention investigates a different approach to the design of a bifocal contact lens. It considers the construction of a bifocal contact lens such that every region of the lens will exhibit the bifocal property. Multiple focal power lenses are known. They consist of plurality of annular rings, the surfaces of which have the same inclinations to the optical axis, in a repetitive alternating pattern, as the individual surfaces of the single focal power lenses they wish to combine.
In the past, these multiple focal power lenses have always been of a stepped Fresnel lens form. That is, one or another of the lens surfaces presents itself as a discontinuous saw-toothed surface. This has allowed for keeping lens thickness small. In camera lenses and in regular spectacle lenses this is an advantage and poses no great problems.
However, if such a surface form were used to construct a bifocal contact lens, the troughs between the annular steps would fill with tears and completely destroy the optics of the contact lens. This can be easily seen with reference to FIG. 2 where we see a cross-sectional view of a stepped Fresnel lens with a tear layer, illustrated by a broken line, adhering to the stepped surface. In cameras and spectacles where the lenses are fairly large, it is often crucial to keep lens thickness small, and that is precisely the point of a stepped Fresnel lens. But to accomplish this, adjacent annular zones A must be separated discontinuously from each other by annular facets I which are parallel to the optical axis so as not to form part of the refractive surface. This construction forms the steps or echelons of a Fresnel lens. However, in the case of a contact lens the refractive surface is actually formed by the tear layer adhering to the anterior lens surface. As shown in FIG. 2, the tears tend to smooth out the discontinuous saw-toothed surface and thereby bring the annular facets I into an inclination which will destroy the quality of the new refractive surface formed by the tear layer.